1. Field of the Invention
The present invention relates to an audio reproducing apparatus and a program.
2. Description of the Related Art
In recent years, for portable audio reproducing apparatuses widely used, a headphone using a three pole terminal is generally used. FIG. 6 shows a block diagram depicting an audio reproducing apparatus and a headphone connected thereto. First, on the headphone side, there are three terminals: an L-channel (ch) terminal, an R-channel (ch) terminal, and a ground terminal shared by the L-channel and the R-channel to which signals carried through the L-channel and the R-channel are fed back. To the L-channel terminal, a speaker part HL is connected through a resistance RXL. Moreover, to the speaker part HL, the ground terminal shared by the R-channel is connected through a resistance RYL and a resistance RCOM. To the R-channel terminal, a speaker part HR is connected through a resistance RXR. Moreover, to the speaker part HR, the ground terminal is connected through a resistance RYR and the resistance RCOM.
On the audio reproducing apparatus side, as similar to the headphone side, there are three terminals: an L-channel terminal, an R-channel terminal, and a ground terminal. In addition, there are two input sources (signal sources) of audio signals for the L-channel and the R-channel. As shown in the drawing, the output audio signals from these signal sources are supplied to the L-channel terminal and the R-channel terminal through analog amplifying parts 21a and 21b, respectively. Furthermore, for example, the ground terminal is grounded through a resistance RZC such as a ferrite bead.
In FIG. 6, a general configuration is shown in which the audio reproducing apparatus is connected to the headphone through three terminals. However, with this configuration, a problem of sound leakage arises between the L-channel and the R-channel. For example, suppose a signal L is carried through the L-channel in the case in which no signals are carried through the R-channel in silence. The flow of the signal L on the headphone side goes through the path from the L-channel terminal to the resistance RXL to the speaker part HL, a sound wave is generated in the speaker part HL, and then the sound wave reaches a connecting point X of the resistance RYL to the resistance RCOM. Since the connecting point X is also connected to the R-channel side, the signal L carries the amount of sound leakage from the connecting point X to the R-channel, which causes a sound leakage. Consequently, a sound wave occurs from the speaker part HR of the R-channel which has to be silent. In addition, even though the operations of the L-channel and the R-channel are performed reversely, a sound leakage occurs in the channel opposite to the channel through which the signal is carried.
Here, the factor that a sound leakage occurs on the R-channel side as described above is because the three terminal configuration is employed in which the L-channel and the R-channel share the ground terminal to which signals from the both channels are fed back, and because the resistance RCOM is connected to the ground terminal on the headphone side and the resistance RZC is connected to the ground terminal on the audio reproducing apparatus side. In the case in which the resistance value of the resistance RCOM is 0Ω and the resistance RZC is not connected, or the resistance value of the resistance RZC is 0Ω, even though the L-channel and the R-channel share the ground terminal, the signals carried through one channel do not cause sound leakage signals on the other channel, and are fed back to the ground terminal, causing no sound leakage. However, it is likely that the resistance RCOM is generated even though a very small amount due to the contact resistance of the ground terminal on the headphone side with the ground terminal on the audio reproducing apparatus side. In addition, the resistance RZC is used in the audio reproducing apparatus for attenuating high frequencies. Since it is likely to damage the analog amplifying parts 21a and 21b unless otherwise the resistance RZC is provided, the resistance RZC is necessary to the audio reproducing apparatus.
In order to control such sound leakage, the applicant proposed a configuration shown in FIG. 7. First, referring to FIG. 7, the ratio between signals carried through the L-channel and the R-channel and the amount of sound leakage is expressed by a sound leakage ratio 1/M. In addition, the discussion will proceed as the sound leakage ratio 1/M is equal in the L-channel and the R-channel. To the audio reproducing apparatus shown in FIG. 7, the configuration of the audio reproducing apparatus shown in FIG. 6 is added with analog amplifying parts 14a and 14b which have a gain equivalent to the sound leakage ratio 1/M for the L-channel and the R-channel.
The analog amplifying part 14a is connected to the connecting point of a signal source L to an analog amplifying part 13a, and connected to an R-channel terminal from the connecting point through the analog amplifying part 14a. Then, the analog amplifying part 14b is connected to a signal source R, and the signal source R is connected to an L-channel terminal through the analog amplifying part 14b. In addition, the configuration on the headphone side is the same as the configuration on the headphone side shown in FIG. 6, omitting the discussion here. In the audio reproducing apparatus in this configuration, for example, the gain of the analog amplifying part 14a is multiplied by the signal L carried through the L-channel, whereby a sound leakage delete signal Lc equivalent to the amount of sound leakage generated in the R-channel can be added to the R-channel terminal on the audio reproducing apparatus side in advance. With this scheme, even though the signal L is carried through the L-channel to cause an amount of sound leakage in the R-channel from the connecting point X, the sound leakage delete signal Lc that cancels it is generated in the R-terminal. Thus, the amount of sound leakage and the sound leakage delete signal Lc are cancelled to each other, a sound wave caused by sound leakage hardly occurs in the speaker part HR, and then the sound leakage can be reduced. Also in the case in which sound leakage is reduced on the L-channel side, the analog amplifying part 14b is used to perform the operation described above in which the L-channel and the R-channel are reversed, and then the sound leakage can be reduced.
However, the configuration of reducing the sound leakage like this has problems below. In other words, in the configuration shown in FIG. 7, since a reduction in sound leakage is implemented with the use of analog devices and circuits, the characteristics might be unstable due to variations in devices. In addition, in FIG. 7, it is essential to add a new circuit such as an analog amplifying part, the addition of the new circuit affects the overall circuit, and it is likely that the computed sound leakage ratio does not sufficiently reduce the sound leakage. In addition, the necessity of this addition increases costs for components, causing problems that the power consumption is increased by the additional device (the circuit) and the circuit scale is increased.
Then, as shown in FIG. 8, the function of the analog amplifying parts 14a and 14b in FIG. 7 is implemented by digital signal processing, whereby the sound leakage can be reduced with no additional analog devices. In other words, the product-sum operation performed in FIG. 7 is performed by digital signal processing in a D-Block 20 shown in FIG. 8, the product-sum operation in which the sound leakage delete signal (data) is added to the L-channel and the R-channel.
In the configuration of the audio reproducing apparatus shown in FIG. 8, the configuration is provided with the D-Block 20, D/A converters 19a and 19b, analog amplifying parts 15a and 15b, an L-channel terminal, a R-channel terminal, a ground terminal and a resistance RZC. First, L-channel audio data is supplied to the D-Block 20. Then, it is supplied from the D-Block 20 to the L-channel terminal through the D/A converter 19a and the analog amplifying part 15a. In addition, R-channel audio data is first supplied to the D-Block 20. Then, it is supplied from the D-Block 20 to the R-channel terminal through the D/A converter 19b and the analog amplifying part 15b. Furthermore, the ground terminal is grounded through the resistance RZC.
Then, in the D-Block 20, sound leakage reducing multipliers 16a and 16b, overflow preventing multipliers 17a and 17b and adders 18a and 18b are provided. In addition, the configuration inside the D-Block 20 is depicted by hardware for convenience. However, in practice, these functions are implemented by digital signal processing.
First, the L-channel audio data is supplied to the overflow preventing multiplier 17a. The overflow preventing multiplier 17a multiplies the L-channel audio data by the coefficient 1−1/M. This is because the sound leakage delete signal is added to the L-channel audio data to be 1+1/M, causing the risk of causing an overflow in the digital area. More specifically, the coefficient 1−1/M is multiplied in advance to make a gain one, and the gain becomes one after the sound leakage delete signal is added to the L-channel audio data, whereby an overflow is prevented. After the L-channel audio data is multiplied by the coefficient 1−1/M in the overflow preventing multiplier 17a as described above, the data is supplied to the adder 18a. 
In addition, the L-channel audio data is supplied to the overflow preventing multiplier 17a, and is branched and supplied to the sound leakage reducing multiplier 16b as well. The sound leakage reducing multiplier 16b is equivalent to the analog amplifying part 14a shown in FIG. 7, which multiplies the L-channel audio data by the gain equivalent to the sound leakage ratio of the L-channel audio data, and supplies the sound leakage delete signal equivalent to the amount of sound leakage of the L-channel audio data to the adder 18b. 
In addition, the R-channel audio data is first supplied to the overflow preventing multiplier 17b. In the overflow preventing multiplier 17b, the process for preventing an overflow is performed as similar to the overflow preventing multiplier 17a described above. Then, the R-channel audio data is supplied to the adder 18b through the overflow preventing multiplier 17b. In addition, the R-channel audio data is supplied to the overflow preventing multiplier 17b, and is branched and supplied to the sound leakage reducing multiplier 16a as well. The sound leakage reducing multiplier 16a is equivalent to the analog amplifying part 14b shown in FIG. 7, which multiplies the R-channel audio data by the gain equivalent to the sound leakage ratio of the R-channel audio data, and supplies the sound leakage delete signal equivalent to the amount of sound leakage of the R-channel audio data to the adder 18a. 
Then, the adder 18a adds the data supplied from the overflow preventing multiplier 17a to the data supplied from the sound leakage reducing multiplier 16a, and supplies it to the D/A converter 19a. In addition, the adder 18b adds the data supplied from the overflow preventing multiplier 17b to the data supplied from the sound leakage reducing multiplier 16b, and supplies it to the D/A converter 19b. In addition, the configuration of the headphone part shown in FIG. 8 is the same as the configuration of the headphone part shown in FIG. 7, omitting the description.
Since the audio reproducing apparatus and the headphone part in the configuration shown in FIG. 8 allow performing the product-sum operation process in the D-Block 20 by digital processing, it is unnecessary to add the analog devices to reduce the sound leakage as shown in FIG. 7.
In the discussion so far, the case is described in which the amount of sound leakage is fixed. In the configuration shown in FIG. 8, for example, in the case in which the sound leakage ratio is changed from the time of setting, such that a user changes headphones, it is necessary to again set the coefficients 1/M and 1−1/M. However, as practical work, it is difficult that a manufacturer receives the audio reproducing apparatus and the headphone from the user to again set the sound leakage ratio.
Then, this scheme is performed in which the sound leakage ratio of each of a plurality of headphones is set in an audio reproducing apparatus in advance, and such a menu facility is provided that a user can select the sound leakage ratio suitable for headphones used by the user in the audio reproducing apparatus, whereby high versatility is provided to the audio reproducing apparatus.
Patent Reference 1: JP-UM-A-6-62622